"seamark:" is a namespace that indicates a feature defined by IHO. IHO is the international organisation responsible for nautical chart standards.

Tags of the form "seamark=..." are not part of this namespace.

All nautical objects tagged within the "seamark:" namespace must include a seamark:type=* tag, which indicates a particular navigational feature that is used in sea charts. For rendering in nautical charts, this tag must be used to indicate the principal symbol to be displayed.